A thoughtful poll, Cristina.
I thought III's end was more "tragic." While ordering Fredo's death at the end of II took a big toll on Michael, he had "cause" and he was was in "control" of it--and control was all-important to Michael. But in III, he thought he was within inches of being totally legit. In the penultimate scene, he's together with his whole family, including Kay, and a bullet intended for him kills his beloved daughter. Not expected, not in control, the final blow.
While I didn't think that III "had" to be made, I welcomed it wholeheartedly. Whatever its flaws--and there were many, many--it served as a denouement to the Corleone saga.
Note to svsg: I don't think that "crime doesn't pay" and "a man has but one destiny" are inimical to each other. I believe they can compliment each other. I'd say you were right: Michael did have one destiny, and he followed it despite having many opportunities to put the criminal life behind him. And I'd also say that his destiny was surrounded by the inevitability that crime doesn't pay. Despite his brilliance, his craftiness, his resiliency, Michael was constantly winning battles--and losing the war. Why? Because winning the battles was in aid of crime--and crime doesn't pay.